Electrolytic recording medium



3,0Z4,l73 Patented Mar. 6, 1962 3,024,173 ELECTROLYTIC RECORDING MEDIUM Gerald Mandel, Elmhurst, N.Y., assignor to Hogan Faximile Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Jan. 26, 1960, Ser. No. 4,629

' 14 Claims. (Cl. 204-2) This invention relates to the art of electro-sensitive recording media and relates particularly to an improved recording medium employing a new marking compound. The invention is concerned especially with a novel electrolytic recording method which does not involve or depend upon lake-forming marking compounds and which does not involve or depend upon the formation of a color lake on the recording medium in order to obtain marking of the medium.

Heretofore, certain polyhydroxyphenolic compounds such as pyrocatechin (i.e., catechol) have been discovered to be capable of serving as marking compounds in aqueous electrolytic recording papers such as disclosed in U.S. Patent No. 2,339,267 to J. V. L. Hogan et a1. and U.S. Patent No. 2,358,839 to E. R. Wagner. Such papers may be employed in electrical recording apparatus such as described in U.S. Patent No. 2,575,959 to J. V. L. Hogan. The method and principle of operation, by which all of the aforementioned polyhydroxyphenolic compounds and their derivatives serve to produce a mark on the recording medium, is described in these patents as involving the formation of a color lake on the medium which constitutes the mark. The aforesaid polyhydroxyphenolic compounds and derivatives are disclosed as being selected for their lake-forming characteristic.

Recording paper of the electrolytic type is generally marked by disposing the paper between a positive metal anode electrode and a negative cathode electrode. The paper is impregnated with an electrolytic solution containing as one ingredient a marking compound. Speaking generally, when a voltage is applied between the electrodes and current flows through the paper, the marking compound is capable of reacting with ions supplied by the anode to form a colored mark on the surface of the paper wherever the anode contacts the paper adjacent to the cathode disposed on the other side of the paper.

In order to prove acceptable for various facsimile and related recording purposes, the recording paper must meet certain important requirements. The paper is preferably white initially and must have the ability to produce marks having a density which is a function, preferably linear, of the current passed through the paper. The color tone obtained should be able to vary uniformly in a range from white (i.e., for zero current flow) through grays (i.e., for currents of intermediate magnitudes) to black (i.e., for high or maximum current magnitude). Marks having a substantially purple, green or brown cast are generally not desirable. There also should be a minimum of bleeding or fringing of the mark so as to maintain sharp detail for the mark. The wet paper, while stored in a special container for extended periods of time prior to use, must remain stable (fresh) without chemical decomposition of its ingredients which would cause discoloration of the paper prior to use and would leave a discolored background after recording.

Moreover, it is very important to employ a recording paper capable of yielding a recorded copy which will remain stable to light and will not streak either during or after recording and after exposure to light for an extended period of time. Similarly, it is desirable to obtain a recorded copy which will remain insensitive to light and will not discolor after standing or even after being repeatedly run through any of the conventional duplicating machines involving exposure to ultra-violet light, such as an Ozalid machine.

Another very important consideration is the provision of a recording paper wherein the marking compound or other active ingredients in the paper will not diffuse or transfer, during or after recording, to adjacent sheets or surfaces of marked or unmarked paper nor to adjacent areas such as the walls of the recorder cabinet or of the room itself. Such transfer frequently results in an objectionable discoloration of the adjacent paper surfaces and walls.

In certain applications difiiculty has been experienced in the past with the use of recording papers employing pyrocatechin (catechol) as the marking compound. An especially troublesome problem in the use of catechol recording papers is the tendency for the catechol to diffuse or transfer to, and to discolor, adjacent sheets of paper and other surfaces adjacent to the recorder during and after recording. In order to obtain marks of full density when using the catechol recording papers disclosed in the aforementioned patents, it has generally been found necessary to provide an electrical heater device adjacent to the paper after it passes from between the electrodes. The presence of such a heater device increases still further the transfer characteristic of the catechol.

In my copending U.S. patent application, Ser. No. 838,860, filed September 9, 1959, now abandoned, and assigned to the same assignee as the aforementioned patents and the present application, I disclose an electrolytic recording paper in which tannic acid replaces catechol as the marking compound and in which a source of citrate ions, such as citric acid, is employed. Such tannic acidcitrate paper avoids the transfer problem presented by catechol paper. However, tannic acid has a distinctly yellow color, and it has been found that both the recording paper and the final recorded copy obtained from the various tannic acid recording papers invariably have a distinctly yellow (rather than white) background. It has also been found that the mark produced in the various tannic acid papers is more purple than it is black.

Another significant observation is made at this point. While it is true that certain prior patents, such as U.S. Patent No. 2,339,267 supra, suggest that various chlorides and bromides may be used as the electrolyte in place of a nitrate, nevertheless subsequent experience through the years has revealed to the art that no chloride or bromide could be successfully employed as the electrolyte in these electrolytic recording media. The reason is that the recorded copy obtained from paper containing any chloride or bromide as the electrolyte, in combination with any specific marking compound (even tannic acid) hitherto employed, was invariably found to become streaked during and after recording. Consequently, to avoid streaking it has been the general practice to utilize a nitrate, such as potassium nitrate, as the electrolyte in recording papers, including the various tannic acid papers. Recording papers containing the various nitrates as electrolytes are unfortunately light-sensitive in that they discolor after exposure to light, particularly ultra-violet rays, during and after recording, such as when the recorded copy is stored or run through an Ozalid duplicator machine or the like.

It is therefore an object of the invention to provide improved electro-responsive graphic recording media.

It is another object to provide electro-sensitive recording media containing as a marking compound a compound which heretofore has not been employed for this purpose.

It is another object to provide a new electrolytic recording method and media which does not depend upon lakeforming compounds nor the formation of color lakes in order to mark the media.

It is another object to provide new electro-sensitive recording media wherein there is no external difiusion or transfer of the ingredients of the media during or after recording.

. A further object is to provide a white electrolytic recordmg medium which does not contain catechol but rather contains a new substantially colorless marking compound which, both before and after recording, does not discolor or otherwise alter the white appearance of the medium.

Another object is to provide an electrolytic recording media which contains a marking compound other than catechol and yet still provides a mark having a color tone which includes a range of gray colors up to and including dense black rather than providing a mark which is more purple, red or brown than it is black.

It is a further object to provide electrolytic recording media which is chemically stable prior to useand which will remain stable to light both during and after recording and which will permit the recorded copy to be exptosed repeatedly to ultraviolet light in a duplicating mac me.

It is an especially important object to provide electrolytic recording media with a new marking compound which is capable of being used with any bromide or chloride as the electrolyte without the medium becoming unstable to light (including ultra-violet light) or streaking during or after recording as heretofore experienced.

Other objects and advantages of the invention will be apparent to those skilled in the art.

In accordance with one aspect of the present invention I have discovered that protocatechuic acid serves as an excellent marking compound in electro-sensitive recording media to accomplish the above and other objects. Heretofore, the use of protocatechuic acid appears to have been restricted to the medical field. Protocatechuic acid is not a lake-forming compound and does not depend upon or involve the formation of a color lake in order to produce a mark in an electrolytic recording medium.

Quite unexpectedly I have also discovered that protocatechuic acid, when employed as a marking compound in an electrolytic recording medium, provides the new and surprising result of making it possible for the first time to employ successfully a bromide or chloride (any bromide or chloride) as the electrolyte without causing streaking during the marking process, thereby avoiding the need for using a nitrate as the electrolyte which itself is sensitive to light.

In accordance with another aspect of the invention there is provided a graphic recording medium comprising a support treated with a quantity of protocatechuic acid as an electro-responsive compound for marking the support.

In accordance with another aspect of the invention there is provided an electrolytic recording method and media in which. the recording medium comprises an inert, porous and preferably white support, such as a sheet of paper having a high wet-strength, which is impregnated with an aqueous solution of protocatechuic acid and an electrolyte. The electrolyte may be a nitrate but preferably is a chloride or a bromide. The impregnant may also include a stabilizer for the recorded color mark. The impregnant may also include an acidifier, and may include one or more antifringing agents.

Recording paper may be impregnated with any of the following formulations which are given by Way of example:

Example No. 1

Sodium chloride ..grams 400 Protocatechuic acid do 70 Thiourea do Water cc 4000 Example N0. 2

Sodium chloride grams 400 Citric acid do 1 10 Protocatechuic acid do 70 Thiourea do 20 Monosodium phosphate do 4 Water cc 4000 pH will be approx. 2.4.

Example N0. 3 Potassium nitrate grams 10 Protocatechuic acid do 2 Thiourea do 0.5 Water cc In the above examples, the quantities given for the protocatechuic acid represent the approximate solubility limit for this material. In Examples No. l and No. 2, the sodium chloride electrolyte may vary from zero to about 1200 grams (the solubility limit) depending upon the degree of electrical conductivity desired, and in Example No. 3 the potassium nitrate electrolyte may similarly vary from zero to 30 grams. In Examples No. 1 and No. 2 the sodium chloride may be replaced, without obtaining streaking, by sodium bromide or by any of the chlorides or bromides of any metal the ions of which do not react with the other components in the solution. Typically, these metals include calcium, potassium, magnesium, cesium, lithium or rubidium. In Example No. 3 the potassium nitrate may be replaced by any of the above chlorides or bromides, and for certain applications where light sensitivity may be tolerated, the potassium nitrate may be replaced by nitrates of the above metals. Similarly in Examples No. l and No. 2 the sodium chloride may be replaced by nitrates of the above metals for those applications where light sensitivity may be permissible.

In the above examples the for the recorded marks and thiourea serves as a stabilizer may be replaced by another suitable stabilizer such as one of the alkyl derivatives of thiourea or dithiobiuret. In Examples No. l and No. 2 the thiourea may be varied approximately from 5 grams, below which instability occurs, up to about 360 grams which is the approximate solubility limit for the thiourea. Similarly in Example No. 3, the thiourea may vary approximately from 0.1 gram up to 9.0 grams.

In Example No. 2 the citric acid serves as an antifringing agent and also as an acidifier. The citric acid may vary preferably from zero to about 50 grams, above which the density of the mark becomes appreciably less. Citric acid may be employed in Example No. l in the same quantity and range. In Example No. 3 an acid such as citric acid or sulfuric acid may be used as an acidifier. In the above examples the pH preferably is in the range from about 1.0 to 3.5. With a pH value above 3.5 the mark is apt to change color. The pH may be less than 1.0 in those applications where a lighter mark is acceptable. In Example No. 2 the monosodium phosphate provides an additional anti-fringing function. It may be varied from zero to about 20 grams, above which the density of the mark becomes appreciably less.

Recording paper may be prepared by passing the porous white paper sheet or other porous support of high wetstrength through a bath prepared in accordance with one of the three above formulae given as examples. Approximately 2.5 to 3.0 cc. of the solution is required per square foot of paper in a typical case. The solution is permitted to distribute itself evenly throughout the paper. The excess impregnant may be removed, as by passing the paper through pressure rolls, so that when ready for use in a recorder machine, the recording paper will have about 40% moisture by weight. In order to retard evaporation, the recording paper is stored in sealed containers until needed.

During the recording process a recorder, such as the type referred to above, may be employed in which an electric current is passed through the recording paper sandwiched between cathode an anode electrodes in contact with opposite surfaces of the paper. The anode may be composed of iron, stainless steel or some other eroding metal capable of reacting with the protocatechuic acid in the solution to form colored complex ions which constitute the recorded color marks on the paper or other support. The recorded paper may then be dried in the air or by some heating device, and the recorded marks will be permanently set. Although not required in order to obtain adequate density of the marks on recording media in accordance with the invention, nevertheless a special heater such as disclosed in US. Patent No. 2,485,678 may be employed with the recorder if desired.

Certain specific combinations, materials and specifications have been given by way of example. Various other modifications may be made. For example, a formulation containing protocatechuic acid as the principal marking compound may also contain, in accordance with the present invention, one or more other marking compounds in addition to obtain any further improvement desired. It will now be apparent to those skilled in the art that various changes and modifications may be made within the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. An electrolytic recording medium comprising an impregnated sheet containing as a marking compound protocatechuic acid in an electrolytically conducting solution.

2. An electrolytic recording medium according to claim 1 in which an acid solution is incorporated therewith.

3. An electrolytic recording medium according to claim 1 in which an acid solution is incorporated therewith, the acidity of the solution being in the range of approximately pH 1 to pH 3.5.

4. An electrolytic recording medium according to claim 1 in which an acid solution is incorporated containing at least one compound selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, and citric acid and its salts. I

5. An electrolytic recording medium according to claim 1 in which a stabilizer is incorporated.

6. An electrolytic recording medium according to claim 1 in which a stabilizer and an acid solution are incorporated.

7. An electrolytic recording medium according to claim 1 in which a stabilizer is incorporated selected from the group consisting of thiourea, alkyl derivatives of thiourea, and dithioubiuret.

8. An electrolytic recording medium according to claim 1 in which is incorporated a compound selected from the group consisting of citric acid and salts thereof, together with a stabilizer selected from the group consisting of thiourea, alkyl derivatives of thiourea, and dithioubiuret.

9. An electrolytic recording medium according to claim 1 in which the electrolyte is selected from the group consisting of metal chlorides and metal bromides.

10. An electrolytic recording mediumaccording to claim 1 in which the electrolyte is selected from the group consisting of chlorides and bromides of sodium, calcium, potassium, lithium, magnesium, rubidium and cesium.

11. An electrolytic recording medium according to claim 1 in which the electrolyte is selected from the group consisting of metal chlorides and metal bromides, and incorporated an acid solution containing at least one compound selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, and citric acid and its salts.

12. An electrolytic recording medium according to claim 1 in which the electrolyte is selected from the group consisting of brornidm and chlorides of sodium, calcium, potassium, lithium, magnesium, rubidium, and cesium, and incorporated an acid solution containing at least one compound selected from the group consisting of sulfuric, nitric, hydrochloric, and citric and its salts, and a stabilizer selected from the group consisting of thiourea, alkyl derivatives of thiourea, and dithioubiuret.

13. An electrolytic recording medium according to claim 1 in which is included a stabilizer and an acid, the impregnant consisting essentially in approximate parts by relative weights of protocatechuic acid 7, electrolyte 40, acid 1, and stabilizer 2.

14. An electrolytic recording medium according to claim 1 in which the components are in an aqueous solution and include a stabilizer, the solution consisting essentially in approximate parts by relative weights of protocatechuic acid 7, sodium chloride 40, citric acid 1, and thiourea 2.

References Cited in the file of this patent UNITED STATESPATENTS 

1. AN ELECTROLYTIC RECORDING MEDIUM COMPRISING AN IMPREGNATED SHEET CONTAINING AS A MARKING COMPOUND PROTOCATECHUIC ACID IN AN ELECTROLYTICALLY CONDUCTING SOLUTION.
 12. AN ELECTROLYTIC RECORDING MEDIUM ACCORDING TO CLAIM 1 IN WHICH THE ELECTROLYTE IS SELECTED FROM THE GROUP CONSISTING OF BROMIDES AND CHLORIDES OF SODIUM, CALCIUM, POTASSIUM, LITHIUM, MAGNESIUM, RUBIDIUM, AND CESIUM, AND INCORPORATED AN ACID SOLUTION CONTAINING AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULFURIC, NITRIC, HYDROCHLORIC, AND CITRIC AND ITS SALTS AND A STABILIZER SELECTED FROM THE GROUP CONSISTING OF THIOUREA, ALKYL DERIVATIVES OF THIOUREA, AND DITHIOUBIURET. 